662 THE VOYAGE OF H.M.S. CHALLENGER. 



cortical shells) are prolonged on the outside into prominent free radial spines. But other 

 spines may also arise from the surface. Very remarkable is the presence of twenty 

 symmetrically disposed radial spines in some forms of Amphitholida (Ampliitholus 

 acanthometra and Amphitholonium acanthonium, &c.). It recalls the twenty radial 

 spines of the Acanthonida, though the laws of distribution in the two similar cases 

 are quite different. 



The cortical shell of the Tholonida remains either simple, or it becomes doubled by 

 formation of an outer veil or envelope. This outer cortical shell or " veil " may either 

 repeat completely the typical form of the inner with its cupolas, or it may form an 

 ellipsoidal envelope without dome-shaped partitions. Its network is either like that of 

 the inner, or it is very delicate and irregular, spider-web like. The connection between 

 the two cortical shells is effected by a variable number of radial beams, often prolonged 

 on the outside into radial spines. 



The growth of the Tholonida by apposition of new chambers is very characteristic, 

 constantly pairs of chambers originating at both opposite poles of the three dimensive 

 axes, firstly on the transverse axis, secondly on the principal axis, thirdly on the sagittal 

 axis. It is possible that by repeated apposition of new chambers the Tholonida are 

 transformed into Discoidea, but commonly the number of domes is restricted, 

 two in the Amphitholida, four in the Staurotholida, six in the Coccotholida. If new 

 chambers in an irregular manner or in a spiral order be opposed to those first formed pairs 

 of chambers, the Tholonida may be transformed into Soreumida, Streblemida, or Lithelida. 

 The phylogenetic connection between these families is probably very complex and very 

 worthy of further research. 



The network of the shell in the Tholonida is spmetimes regular, with circular (often 

 hexagonally framed) pores of equal size, sometimes irregular, with roundish pores of unequal 

 size. Commonly the shell is thick and compact ; in those genera in which the cortical 

 shell is doubled, the inner is commonly compact, the outer a very delicate veil. 



The central capsule of the Tholonida is originally always enclosed by the central 

 chamber, and like this of lentelliptical form, a triaxial ellipsoid. With the apposition of 

 cupolas the growing central capsule may form dome-shaped protuberances which enter into 

 the former. So we may find in the Amphitholida a central capsule with three joints 

 (separated by two annular parallel constrictions), in the Staurotholida a cross-shaped capsule 

 (with four caps around the central lentellipsis), in the Cubotholida a clustered capsule with 

 six caps, surrounding six sides of the cubical central mass. In the Coccotholida (with 

 medullary shell in the central chamber) the latter encloses in the centre the nucleus of 

 the cell. The calymma, or the jelly-veil between the central capsule and the enveloping 

 shell, probably always exhibits annular constrictions, corresponding to those which 

 separate the cupolas of the shell. All these anatomical peculiarities in the Tholonida 

 require a further accurate study. 



